Biological barriers that limit delivery of imaging and therapeutic agents to the tumour pose a technological challenge. One of those barriers, the vascular endothelium, uses vesicles called caveolae to transport proteins across the cell. Annexin A1 (AnnA1) was enriched in the caveolae at the blood–tumour interface of both human and mouse tumours, so the authors generated a fluorescent-conjugated AnnA1 antibody. When injected intravenously in a mouse model of breast cancer, the antibody rapidly crossed the blood vessels and was taken up by tumours, where it remained for over one hour.